This invention relates to condensers that collect radiation and deliver it to a ring field. More particularly, this condenser collects radiation, here soft x-rays, from a small, incoherent source and couples it to the ring field of a camera designed for projection lithography.
Projection lithography is a powerful and essential tool for microelectronic processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. Soft x-rays are now at the forefront of research in the efforts to achieve the desired feature sizes. This radiation has its own problems however. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that most x-ray reflectors have efficiencies of only about 60%. This alone dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces which can image with acuity only along a narrow arc or ring field 48. Such cameras then use the ring field to scan a reflective mask 46 and translate that onto the wafer 16 for processing. Although cameras can be designed to do this, there is as yet no available condenser system which can efficiently couple the soft x-rays from a source to the ring field required by this type of camera.
Further, full field imaging, as opposed to ring field, requires severely aspheric mirrors. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at these wavelengths.